1. Field of Invention
The present invention relates to a method and structure for controlling plasma uniformity in plasma-based surface processing applications. The present invention includes controlling plasma uniformity in a high density plasma deposition and etching apparatus such as described in U.S. Pat. No. 5,429,070 which is incorporated herein in its entirety by reference.
2. Related Art
Applications for the use of plasmas are widespread, and a particular application is in the area of semiconductor fabrication. Plasmas are used as dry etchants in both blanket and patterned etches. Such etches can exhibit good anisotropic and selective etching qualities, and particular plasma etches, such as reactive-ion etches, allow the etching of fine patterns with good dimensional control.
Uniformity is a critical issue in plasma etch systems. In a typical application, a layer of material is etched with the intention of stopping the etch on the layer underneath. As a general rule, plasma contact with the underlying layer is to be minimized. Furthermore, the profile of etched features depends critically upon the etch rate and the time (known as the over etch time) that the plasma is in contact with the feature after definition of the feature by the etching process. Nonuniform plasmas degrade the uniformity of the etch rate over the entire wafer surface, requiring increased over etch times to ensure complete removal of all material desired to be etched. Because the thickness of the layers is a constant for a given device, independent of the wafer size, the etch rate uniformity across the wafer becomes an increasingly rigorous criterion as wafer/substrate dimensions increase. The present de facto industry standard is a 5% uniformity variation from the mean at three (3) standard deviations over the wafer surface. Increases in the diameter of semiconductor wafers and further decreases in the thickness of thin films have made the need for uniform etching, and hence, uniform plasma densities even more important.